Hops provide bitterness to balance the sweetness of Malt when making beer, adds flavoring oils and aromas, and also helps to stabilize and preserve beer. Hops used in brewing comes from the flowers of a plant called Humulus lupulus. The hop plant is a perrenial spiraling vine that requires most soil. The flowers of the hops, called cones are dried before use. These flowers are usually green in color with yellow lupulin glands between the petals that provide many of the oils.

Types of Hops

Loose or Leaf Hops - Hops in its most natural form. Leaf hops float, provide a nice filter bed when siphoning, and are excellent when fresh. Unfortunately these hops are also most susceptible to exposure to air and oxidization, which means their quality will decline more rapidly unless vacuum sealed in a oxygen barrier bag.

Plug Hops - Dried and compressed hop cones. When hydrated these are essentially the same as whole hops, but will store better.

Pellet Hops - Perhaps the most widely available to the home brewer, these hops are dried, chopped and compressed into tiny pellets. They store well, and are easy to measure in small quantities. The chopping and compressing can release some of the lupulin glands to burst losing some aromatic oils.

Measuring Bitterness

Hop bitterness is measured in International Bitterness Units or IBU's. One IBU is one part per million of isohumulone which is a bittering (alpha) acid. IBU's can be estimated when brewing a beer by several different formulas, the most popular of which are the Tinseth, Rager and Garetz formulas. IBU's for light beers are generally in the 10-20 range, while dark flavorful beers such as stouts may have an IBU as high as 50. Some barley wines have IBU values of 100 or more to offset the extreme malty sweetness of the beer. See the Beer Styles BJCP guide for some typical IBU ranges for different styles of beer.

Bittering Resins and Oils

Bittiness in beer is provided by resins released by the hops. The battering resins of the hops are isomerized (rearranged) during the boil. Insoluable alpha acids (α-acids) are isomerized by the boil into more soluble and stable alpha acids. As the boil time increases, the bitterness released also increases. These alpha acids provide the majority of the bitterness in finished beer, as estimated by the Alpha Acid Units (AAUs) or Homebrew Bitterness Units (HBU's). A second resin called beta acid also provides some bitterness. The amount of bettering coming from beta acids depend on the degree of oxidation, or, how much oxygen the hops have been exposed to over time.

Hop oil compounds provide flavor, aroma and preservative qualities. They are soluble in water and easily boiled off. Dry hopping will allow one to impart these properties without the risk of boiling-them-off.

Hops Storage

The alpha and beta acids in hops are both vulnerable to oxidation which will decrease their effectiveness. Hops will degrade faster at temperatures above freezing. Hops should be refrigerated in your freezer, and sealed in an airtight container (ideally vacuum packed foil oxygen barrier container) to prevent oxidation.

Hops Usage

Hops can be used at many stages in the brewing process:

Boil Hops - Hops used during the main boil to add bitterness and flavor to the beer.

Aroma Hops - Hops added at the end of the boil to release aroma. Many aromatic oils in hops will boil off after a period of time, so hops added at the end of the boil maintain many aromatic qualities.

Dry Hops - Hops added in the secondary fermentation, usually a day or two before bottling, primarily for aroma.

Mash Hops - Hops added to the mash tun, effectively get steeped and can add some flavor though they usually contribute little to the bitterness of the beer.

First Wort Hops - Hops added to the boiler as the wort is first being sparged. These hops effectively get steeped and then boiled with the main boil. First wort hopping results in a better blending of hops flavor with the wort though it generally reduces hop utilization slightly when compared to traditional boiled hops.